The present invention relates to a method and apparatus for strengthening pressure resistance of a hollow metallic structure such as, for example, a pipe conveying high pressure fluid.
In, for example, JP-B53-38246 (1987), a thermal treatment is applied to a metallic pipe in a plant which deals with high pressure fluid in order to increase reliability in the transportation of high pressure fluid by the steel pipe.
In the above-referenced thermal treatment arrangement, an apparatus is proposed wherein valves partition a interior of a portion of a steel pipe to be thermally treated in a liquid tight manner, and a heating coil of a high frequency induction heater is provided around the portion of the steel pipe, or an electrode is connected to a power source and is attached to the steel pipe so as to improve a resistance to stress corrosion cracking.
In the last-mentioned apparatus, a coolant, enclosed in the partitioned portion of the steel pipe by valves, becomes stagnant in the portion of the steel pipe being treated. By virtue of the existence of the coolant in the steel pipe, an outer side of the portion of the steel pipe being treated is heated with the heater, while an inner side is cooled with the coolant which removes the heat from the inner surface. The heating is continued until a temperature differential between the outer side and the inner side of the steel pipe reaches a temperature differential which causes a tensile yielding stress in the inner side of the steel pipe and a compressive yielding stress in the outer side of the steel pipe. The heating is subsequently stopped and a part of the strain is released following a stress-strain curve which is different from a stress-strain curve of a steel pipe obtained during the heating because the steel pipe reached a yielding region once before in the thermal treatment, and as a result, a balanced condition is achieved wherein a compressive residual stress is provided in the inner side of the steel pipe and a tensile residual stress is provided in the outer side of the steel pipe, and absolute values of the compressive residual stress and the tensile residual stresses are nearly equal.
By providing the compressive residual stress in the inner side of the steel pipe by the method described above, a pressure resistance property of the steel pipe is strengthened with an offset of a tensile stress in the inner side of the steel pipe which is caused by an internal pressure of a fluid in plant operation with the provided compressive residual stress in the inner side of the steel pipe.
Another example of the prior art to strengthen a pressure resistance property of a steel pipe by providing a compressive residual stress in an inner side of the steel pipe is disclosed in the JP-A-57-177924 (1982), wherein a volume of ice is used over a given time period to form ice plugs by freezing the coolant in side regions adjacent to both sides of a portion of the steel pipe being treated for raising a internal pressure in the pipe to cause a stress higher than a yielding stress of the steel pipe and causing a yielding of the steel pipe which provides a compressive residual stress in the inner side of the pipe after releasing of the internal pressure with thawing of ice by heating.
In the JP-B-53-38246 proposal, a strength of the tensile residual stress which is provided in the outer side of the steel pipe is as large as being equal to a strength of the compressive residual stress in the inner side of the steel pipe, and when an internal high pressure is applied in the steel pipe, a stress caused by the internal high pressure to the steel pipe is tensile stress in all regions through the wall of the steel pipe in a thickness direction from the inner side to the outer side of the steel pipe. Therefore, when a large tensile residual stress is provided in the outer side of the steel pipe, an effect of a tensile stress in the outer side of the steel pipe, larger than a yielding stress of a material of the steel pipe, results because of a superimposition of an additive tensile stress caused by high internal pressure loading in plant operation on the tensile residual stress in the outer side of the steel pipe thereby easily causing a larger tensile stress than the yielding stress of the material of the pipe. An excessively large tensile stress as described above results in a breakage of the steel pipe from the outer side.
In JP-A-57-177924, with an expanding of a steel pipe in cold condition, in an installed steel pipe which has been used in a plant once before being subjected to thermal treatment or even a new steel pipe, when the new steel pipe is defective at an inner side thereof, a defect existing in the inner side of the steel pipe is enhanced with the force of expanding of the steel pipe easier in a cold condition than in a high temperature condition, and a possibility of increasing a breakage factor arises. Further, it is also possible for an increase in the breakage to arise because the higher pressure is used for the expanding of the steel pipe is performed only by an internal pressure.